Water electrolysis system

ABSTRACT

In a water electrolysis system having an anode catalyst layer containing anode catalyst and a cathode catalyst layer containing cathode catalyst tightly attached, respectively, to each surface of a solid polymer electrolyte membrane comprising a cation exchange membrane, wherein at least one catalyst layer of said anode catalyst layer and cathode catalyst layer comprises a porous structure of anode catalyst or cathode catalyst dispersed in fluorine resin containing resin, featuring the surface of the anode catalyst layer or the cathode catalyst layer being hydrophobized and the water contact angle with the surface of the anode catalyst layer or the cathode catalyst layer of said porous structure being 90 degrees or more, whereby the transfer of gas to the counter electrode can be significantly suppressed, gas purity and current efficiency be improved, and safety operation of the electrolysis system be secured, without a major change in configuration of the water electrolysis system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a water electrolysis system for generatingoxygen and hydrogen or for generating ozone by water electrolysis inwhich an anode and a cathode are tightly attached, respectively, to eachsurface of a solid polymer electrolyte membrane comprising a cationexchange membrane, enabling hydrogen gas or oxygen gas generated at thecathode or the anode to be suppressed against transferring to thecounter electrode through the cation exchange membrane, the purity ofthe gases to be enhanced, and safe electrolysis operation to be securedfor a long time.

2. Description of the Related Art

An electrolysis system which is composed by attaching an anode and acathode to each surface of a solid polymer electrolyte membranecomprising a cation exchange membrane has been widely used as a superiorenergy-effective electrolysis system for its advantages including a lowelectrolytic voltage, availability of direct electrolysis of pure waterwhich is not electrolyzed by an ordinary electrolysis process because ofits low electrical conductivity and compact equipment design, andcommercialized as a water electrolysis system for generatingelectrolytic oxygen and hydrogen. The electrolysis system is alsocommercialized as a water electrolysis system for ozone generation whichutilizes a fluorinated ion exchange membrane as an electrolyte havingunique properties.

In such a water electrolysis system, attaching method between a cationexchange membrane and electrode catalyst or between electrode catalystand a current collector is highly important for effective utilization ofadvantages of the present electrolysis system. The method is classifiedinto two groups.

One is the group in which electrode catalyst is loaded directly on thesurface of a cation exchange membrane, wherein the cation exchangemembrane on which metal salt has been adsorbed is made contact withreducing agent in order to deposit the metal directly on the surface ofthe cation exchange membrane. (Refer to JP 6-41639) The cation exchangemembrane/electrode catalyst structure prepared by this method showssatisfactory contact between the cation exchange membrane and electrodecatalyst, but has such problems that the electrode catalyst layer isextremely thin; the electrode catalyst tends to have the concentrationdistribution of electrode catalyst in the direction of the cationexchange membrane thickness depending on the adsorption conditions suchas concentration or temperature of metal salt solution; uniform contactwith the current collector is difficult because formed electrodecatalyst layer is thin; applicable electrode catalyst is limited tometals that are formable by using reducing agent.

The other is the group in which electrode catalyst is loaded on thesurface of the current collector, showing no problems observed as in thefirst group, allowing a wider selection of electrode catalyst andenabling to form the electrode catalyst layer as thick as several tensof μm. The loading methods include a method to load electrode catalystlayer comprising metals or metal oxides directly on the currentcollector by such means as electrolytic plating, CVD, and sputtering; amethod to load electrode catalyst layer by applying, followed by drying,paste of electrode catalyst powder mixed with resin or organic solventand a method to load electrode catalyst layer comprising metal oxides byapplying metal salt solution on the current collector, followed bythermal decomposition.

When a solution with a large specific resistance like pure water iselectrolyzed by a water electrolysis system composed of the cationexchange membrane bonded with the electrode catalyst and the electrodecatalyst bonded with current collector by said methods, its electrolyticreactions proceed chiefly on the surfaces interfacing three phases ofthe cation exchange membrane/electrode catalyst/solution (three-phaseboundary). For instance, when iridium is applied as electrode catalystof the anode and platinum-loaded carbon as electrode catalyst of thecathode, oxygen generation reaction proceeds at the anode and hydrogengeneration reaction proceeds at the cathode, and generated gas bubblesgrow at the three-phase boundary.

The gas bubbles which have grown to a certain size at the three-phaseboundary are vented outside the electrolysis system from the three-phaseboundary through the internal portion of the current collector; whereas,a portion of the generated gas moves to the counter electrode throughthe cation exchange membrane by concentration diffusion which appliespressure inside bubbles as driving force. For instance, in a zero-gapwater electrolysis system, such phenomenon occurs that hydrogengenerated at the three-phase boundary of cathode catalyst/cationexchange membrane/water reaches the anode, which is the counterelectrode through the cation exchange membrane and is vented outside ofthe cell as a mixture with oxygen gas.

Gas transfer to the counter electrode leads to performance deteriorationof the water electrolysis system in terms of decrease in purity of thegas produced as the counter electrode gas and decrease in currentefficiency of gas production. Moreover, in the water electrolysis systemwhere oxygen and hydrogen are generated, provisions of monitor units andoperational vigilance against intrusion of the counter electrode gas arerequired for safety performance of the water electrolysis system sincehydrogen and oxygen gas mixture may exceed a lower explosion limit as aresult of transfer of the counter electrode gas. The relation of abubble size vs. a surface tension of solution is expressed byYoung-Laplace Equation Pg−P1=2γ/r (where, Pg: Pressure inside bubble,P1: Solution pressure, γ: Surface tension of solution, r: Radius ofbubble). According to this equation, when the solution pressure isconstant, smaller the size of bubble, pressure inside bubble thatconstitutes equilibrium will increase and the gas transfer driving forceto the counter electrode will be intensified.

In the present invention, in order to obtain a water electrolysis systemwhich enhances the purity of electrolytically generated gas, maintainshigh current efficiency and safety performance even for a long timeoperation, discussions have been made on hydrophilic/hydrophobic natureon the side of electrode catalyst in an attempt to suppress the amountof counter electrode gas which passes through the cation exchangemembrane.

As a result, the inventors of the present invention have conceived ameans to decrease the pressure inside bubble without changing surfacetension of water in the water electrolysis system that if electrodecatalyst with sufficient hydrophobic nature is used, a bubble attachesto electrode catalyst with a wide contact area without forming a bubblecontrolled by the surface tension of water, because a bubble contactswater with a large surface tension at the time of forming andconstitutes pressure equilibrium between pressure inside bubble and thesurface tension of solution around the bubble, as expressed byYoung-Laplace Equation, but at the same time the bubble contacts theelectrode catalyst and the cation exchange membrane.

In addition, the inventors of the present invention have found that atthis time, the pressure inside bubble decreases as it evades control bythe surface tension of water, and the driving force for the gas totransfer to the counter electrode decreases, eventually reducing theamount of gas transfer, and especially that by making the electrodecatalyst layer on the cathode side hydrophobic, transfer of hydrogenwhose nature is easy in concentration diffusion because of its smallmolecular size can be suppressed significantly.

SUMMARY OF THE INVENTION

In order to solve said problems, the present invention has constructed awater electrolysis system having an anode catalyst layer containinganode catalyst and a cathode catalyst layer containing cathode catalysttightly attached, respectively, to each surface of a solid polymerelectrolyte membrane comprising a cation exchange membrane, wherein atleast one catalyst layer of said anode catalyst layer and cathodecatalyst layer comprises a porous structure of anode catalyst or cathodecatalyst dispersed in fluorine resin containing resin, featuring thesurface of the anode catalyst layer or the cathode catalyst layer beinghydrophobized and the water contact angle with the surface of the anodecatalyst layer or the cathode catalyst layer of said porous structurebeing 90 degrees or more.

The second method to solve said problems applies polytetrafluoroethyleneas fluorine resin to be used for said porous structure in said waterelectrolysis system.

The third method to solve said problems applies a cathode catalyst layercontaining cathode catalyst comprising said porous structure in saidwater electrolysis system.

The fourth method to solve said problems applies a porous structurecomprising said cathode catalyst layer with platinum or platinum-loadedcarbon grains dispersed in fluorine resin containing resin in said waterelectrolysis system.

The fifth method to solve said problems applies a porous structurecomprising a porous metal plate or a sintered sheet of metallic fiberhaving on its surface anode catalyst containing lead dioxide or iridiumin said anode catalyst layer in said water electrolysis system.

The sixth method to solve said problems applies a catalyst layercomprising said porous structure with a fluorine resin layer coated onthe surface of said cathode catalyst layer in said water electrolysissystem.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of the water electrolysis system by thepresent invention.

FIG. 2 illustrates the contact angle of water.

FIG. 3 illustrates a relation between the contact angles of water vs.concentration of hydrogen in the anode gas.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The following is an explanation of the present invention in reference tothe water electrolysis system illustrated in FIG. 1.

In the electrolysis system by the present invention, 1 indicates thesolid polymer electrolyte membrane comprising the cation exchangemembrane, for which conventionally known cation exchange membranes arewidely applicable, but a preferable membrane is perfluorosulfonic acidcation exchange membrane, chemically stable with a sulfonic acid group.On the anode side surface of the solid polymer electrolyte membrane 1,the anode current collector loaded with the anode catalyst layer 2containing anode catalyst on its surface or the anode substrate 3 arearranged in tight contact. The anode current collector or the anodesubstrate 3 has electrical conductivity and is corrosion resistant tooxidation, as well, allowing a structure sufficiently capable of ventinggenerated gas and circulating electrolyte and providing availability toapply porous material, mesh, fiber, and foam with metal substratesincluding titanium, tantalum, niobium and zirconium.

For the anode catalyst comprising the anode catalyst layer 2, anyconventionally known materials are applicable for oxygen generation,such as iridium and its oxides with a low oxygen overvoltage. For ozonegeneration, materials with a high oxygen overvoltage, such as leaddioxide and electrically conductive diamond are applicable.

The anode catalyst layer 2 comprises porous structure of said anodecatalyst dispersed in fluorine resin containing resin, available to beloaded on the anode current collector, the anode substrate 3 by acoating method or hot press process, or to be used in sheet form of amixture with binder components like fluorine resin or Nafion (RegisteredTrade Mark of Du Pont) solution. In the above process, suchconsiderations as the surface of the anode catalyst layer 2 ishydrophobized, applied composition is determined to obtain the watercontact angle being 90 degrees or more, especially the dispersion ofpolytetrafluoroethylene (PTFE), a material of high hydrophobicity,functions effectively to the top surface, are paid for configuration sothat gas transfer is significantly suppressed, gas purity and currentefficiency are improved, safety of the electrolysis system is securedwithout any major change in construction of the water electrolysissystem. Incidentally, the anode catalyst layer 2 can be loaded on thesubstrate of porous metal plate or sintered sheet of metallic fiber.Also, the anode catalyst layer 2 can be formed by such processes aselectrolytic plating, thermal decomposition, coating, and hot press.

As fluorine resin used for said porous structure, various types offluorine resin are applicable, but polytetrafluoroethylene (PTFE) ispreferable.

Said anode catalyst layer 2 can be tightly attached to the surface ofsaid solid polymer electrolyte membrane 1 by the hot press processinstead of being loaded on the anode current collector or the anodesubstrate 3.

On the cathode side surface of the solid polymer electrolyte membrane 1,the cathode current collector or the cathode substrate 5 loaded with thecathode catalyst layer 4 containing cathode catalyst on its surface arearranged in tight contact. As the material for the cathode currentcollector or the cathode substrate 5, porous materials, same as theanode current collector or the anode substrate, of nickel,stainless-steel, zirconium or carbon are applicable. For the cathodecatalyst composing of the cathode catalyst layer 4, platinum, platinumblack or platinum-loaded carbon, having a low hydrogen overvoltage ispreferable.

The cathode catalyst layer 4 comprises porous structure of said cathodecatalyst dispersed in fluorine resin containing resin, available to beloaded on the cathode current collector, the cathode substrate 5, or thebase material by a coating method or hot press process, or to be used insheet of mixture with binder components like fluorine resin or Nafion(Registered Trade Mark of Du Pont) solution.

In the above process, such considerations as the surface of the cathodecatalyst layer 4 is hydrophobized, applied composition is determined toobtain the water contact angle being 90 degrees or more, especially thedispersion of polytetrafluoroethylene (PTFE), a material of highhydrophobicity functions most effectively on the top surface are paidfor configuration so that gas transfer is significantly suppressed, gaspurity and current efficiency are improved, safety of the electrolysissystem is secured without any major change in construction of the waterelectrolysis system. Incidentally, the cathode catalyst layer 4 can beloaded on the substrate of porous metal plate or sintered sheet ofmetallic fiber. Also, the cathode catalyst layer 4 can be formed by suchprocesses as electrolytic plating, thermal decomposition, coating, andhot press.

As fluorine resin used for said porous structure, various types offluorine resin are applicable, but polytetrafluoroethylene (PTFE) ispreferable.

In addition, the surface of the cathode catalyst layer 4 ishydrophobized by another layer of fluorine resin provided on the surfaceof said cathode catalyst layer 4, thus enabling the water contact angleto be 90 degrees or more.

Said cathode catalyst layer 4 can be tightly attached to the surface ofsaid solid polymer electrolyte membrane 1 by the hot press processinstead of being loaded on the cathode current collector or the cathodesubstrate 5.

Composition must be prepared and construction must be arranged so thatat least one of the anode catalyst layer 2 and the cathode catalystlayer 4 comprises porous structure of said anode catalyst or saidcathode catalyst dispersed in fluorine resin containing resin and thesurface of the anode catalyst layer 2 or the cathode catalyst layer 4 ishydrophobized so that the water contact angle becomes 90 degrees ormore, especially the dispersion of polytetrafluoroethylene (PTFE), amaterial of high hydrophobicity, functions effectively on the topsurface.

As mentioned above, when the surface of the anode catalyst layer 2 orthe cathode catalyst layer 4 is hydrophobized and the water contactangle is made to be 90 degrees or more, the transfer of gas to thecounter electrode can be significantly suppressed, gas purity andcurrent efficiency be improved, and safety operation of the electrolysissystem be secured, without a major change in configuration of the waterelectrolysis system. However, when the water contact angle was below 90degrees, these effects were not sufficient, unable to perform expectedsuppression of gas transfer and improvement of gas purity.

The water contact angle with the surface of the anode catalyst layer 2or the cathode catalyst layer 4 is a value used as an index to indicatehydrophilic and hydrophobic properties of the solid surface. As shown inFIG. 2, the figure of contact angle is obtained by measuring the angleformed by the surface of the aqueous droplet formed on the solidsubstance and the surface of the solid substance using a protractor or acontact angle meter. It is generally said that the smaller the contactangle, the larger the hydrophilic property, and the larger the contactangle, the larger the hydrophobic property or water repellency. Forinstance, Teflon (registered trademark), a typical hydrophobic materialwhose terminating group is covered with fluorine has a water contactangle of 114 degrees, and silicone resin, also known as highlyhydrophobic material has high values as 90-110 degrees; whereas phenolresin having hydroxyl group, which has a high affinity with water, onits surface indicates relatively hydrophilic property at 60 degrees,even if it is a type of resin.

Metals or glasses covered with oxides on their surface generallyindicate low values; for instance, the contact angle for glass having aclean surface without organic contamination shows 5 degrees or below,the measurement being utilized to evaluate the contamination degrees ofglass surface.

FIG. 2 illustrates the detailed measurement method of the water contactangle θ. The contact angle is expressed as θ formed by the tangent Y ofdroplet D and the base material S at the intersection X of the basematerial S and droplet D. When the contact angle θ is measured visuallyusing a contact angle meter, it is difficult to obtain tangent Yaccurately and therefore, the angle θ₁ formed by the line connecting theintersection X to the vertex Z of droplet D and the base material S ismeasured. Assuming that the droplet D is a part of arc, Equation θ=2θ₁is geometrically true; then, the contact angle θ is easily obtained evenin visual observations. The contact angles in the examples and thecomparative examples in the present specifications are measured by thismethod.

Accordingly, when only the anode catalyst layer 2 is catalyst layercomprising said porous structure and the water contact angle with thesurface is 90 degrees or more, the transfer of gas on the anode side tothe cathode side can be significantly suppressed, the gas purity on thecathode side and current efficiency can be improved and the operationalsafety of the electrolysis system can be secured; when only the cathodecatalyst layer 4 is configured as above-mentioned, the transfer of gason the cathode side to the anode side can be significantly suppressed,the gas purity on the anode side and current efficiency can be improvedand the operational safety of the electrolysis system can be secured;when both of the anode catalyst layer 2 and the cathode catalyst layer 4are configured as above-mentioned, the gas purity on both the anode sideand the cathode side and current efficiency can be improved and theoperational safety of the electrolysis system can be secured.

The following explanations are about the examples and the comparativeexamples of the present invention. The present invention, however, isnot limited to these examples.

EXAMPLE 1

A sintered sheet of titanium fiber (manufactured by Tokyo Rope Mfg. Co.,Ltd.), 1 mm thick, was washed with neutral detergent for degreasing andsubject to pretreatment by acid pickling with 20 wt % hydrochloric acidsolution for one minute at 50 degrees Celsius; then, on the saidsintered sheet of titanium fiber, a coating comprisingplatinum-titanium-tantalum (25-60-15 mol %) was formed by the thermaldecomposition method; and thus the anode current collector or the anodesubstrate with an underlayer on the surface is prepared.

Using said anode current collector or anode substrate as the anode, and400 g/l of lead nitrate solution as electrolyte, electrolysis wasperformed for 60 minutes at 60 degrees Celsius at the current density of1 A/dm² to form a coating layer of β—lead dioxide, which is anodecatalyst, on the anode current collector or the anode substrate surface.

A commercially available perfluorosulfonic acid type cation exchangemembrane (Registered Trade Mark of Du Pont: Nafion 117, manufactured byDu Pont) was immersed in boiled pure water for 30 minutes forwater-swelling treatment and used as a cation exchange membrane.

On the other hand, Cathode Sheet A was prepared in such way that PTFEdispersion (manufactured by Mitsui DuPont Fluorochemical Co., Ltd. 30-J)was mixed with aqueous dispersion in which platinum-loaded carboncatalyst is dispersed, followed by drying, and to this mixture, solventnaphtha was added and kneaded, followed by rolling, drying, andsintering to form Cathode Sheet A in porous structure comprising 40 Wt.% of PTFE, 60 Wt. % of platinum-loaded carbon catalyst, with themembrane thickness of 120 μm, and porosity of 55%.

The contact angle of Cathode Sheet A surface was measured with a contactangle meter (manufactured by Elma Model No. G-1), the result was 92degrees.

These elements and a sintered sheet of stainless-steel fiber(manufactured by Tokyo Rope Mfg. Co., Ltd.), which was a 2.5 mm thickcathode current collector, were assembled to a titanium electrolysissystem in the order of Anode Compartment/Anode Current Collector/LeadDioxide Surface/Cation Exchange Membrane/Cathode Sheet A/Cathode currentcollector/Cathode Compartment, and pure water electrolysis wasconducted, while pure water, as electrolyte, being cooled. Then, the gasmixture of ozone and oxygen was generated at the anode and hydrogen gaswas formed at the cathode; the concentration of ozone in generated anodegas: 11.0 Vol. %, concentration of hydrogen gas in the anode gas: 0.05Vol. %, and the cell voltage: 3.3 v.

The electrolytic conditions include current density: 1 A/cm2 andelectrolyte temperature: 30±5 degrees Celsius. Whereas, the electrolyticconditions in the following examples and comparative examples are allidentical to those of Example 1.

EXAMPLE 2

Cathode Sheet B was prepared in such way that an aqueous dispersion ofPTFE dispersion and platinum-loaded carbon catalyst was applied with abrush on carbon paper (100 μm thick) surface, followed by drying, andthis process was repeated three times to form Cathode Sheet B in porousstructure comprising a carbon paper substrate of 110 μm thick. Thecontact angle with the catalyst coating surface of Cathode Sheet B was95 degrees.

The electrolysis test was conducted as in Example 1, the results ofwhich illustrated the concentration of ozone in the anode gas: 11.4 Vol.%, concentration of hydrogen gas in the anode gas: 0.10 Vol. %, and thecell voltage: 3.3 v.

EXAMPLE 3

A sintered sheet of titanium fiber, 1 mm thick, was washed with neutraldetergent for degreasing and subject to pretreatment by acid picklingwith 20 wt % hydrochloric acid solution for one minute at 50 degreesCelsius; then, on the said sintered sheet of titanium fiber, iridiumdispersion prepared by dispersing iridium powder (under 200 mesh) inPTFE dispersion was applied with a brush until the final coating amountreached 250 g/m2 as iridium, followed by drying and thus, an iridiumcoated anode with a sintered sheet of titanium fiber was obtained.

The electrolysis test was conducted as in Example 1, wherein oxygengenerated at the anode and hydrogen generated at the cathode and theresults of which illustrated the concentration of hydrogen gas in theanode gas: 0.07 Vol. %, and the cell voltage: 2.5 v. For the cathode,Cathode Sheet A in porous structure was used, wherein the contact anglewith the catalyst coating surface was 95 degrees

COMPARATIVE EXAMPLE 1

Cathode Sheet C was prepared in such way that Nafion (Registered TradeMark of Du Pont) solution (manufactured by Sigma-Aldrich Japan K.K.) wasadded to a dispersion prepared by dispersing PTFE dispersion andplatinum-loaded carbon catalyst in ethanol, and resultant dispersion wasapplied with a brush on the surface of carbon paper, 100 μm thick,followed by drying, and this process was repeated five times to formCathode Sheet C comprising a carbon paper substrate. The contact anglewith the catalyst coating surface of Cathode Sheet C with a porousstructure was 77 degrees.

The electrolysis test was conducted as in Example 1, the results ofwhich illustrated the concentration of ozone in the anode gas: 11.4 Vol.%, the concentration of hydrogen gas in the anode gas: 0.55 Vol. %, andthe cell voltage: 3.2 v.

COMPARATIVE EXAMPLE 2

Cathode Sheet D was prepared in such way that Nafion ( Registered TradeMark of Du Pont) solution (manufactured by Sigma-Aldrich Japan K.K.) wasadded to a dispersion prepared by dispersing PTFE dispersion andplatinum-loaded carbon catalyst in ethanol, and resultant dispersion wasapplied with a brush on a sintered sheet of stainless-steel fiber, 0.5mm thick (manufactured by Nippon Seisen Co., Ltd.), followed by drying,and this process was repeated five times to form Cathode Sheet Dcomprising a sintered sheet of stainless-steel fiber substrate. Thecontact angle with the catalyst coating surface of Cathode Sheet D inporous structure was 45 degrees.

The electrolysis test was conducted as in Example 1, the results ofwhich illustrated the concentration of ozone in the anode gas: 11.0 Vol.%, the concentration of hydrogen gas in the anode gas: 0.84 Vol. %, andthe cell voltage: 3.1 v.

COMPARATIVE EXAMPLE 3

Cathode catalyst of platinum coated carbon was prepared in such way thatchloroplatinic acid was dissolved in isopropyl alcohol to become 50 g/Las platinum and the resultant solution was applied with a brush on oneside of a porous carbon plate of 2 mm in thickness, followed by thermaldecomposition in reducing flame. The contact angle with the platinumcoated surface of the platinum carbon cathode was 28 degrees.

The electrolysis test was conducted as in Example 1, the results ofwhich illustrated the concentration of ozone in the anode gas: 11.0 Vol.%, the concentration of hydrogen gas in the anode gas: 1.5 Vol. %, andthe cell voltage: 3.4 v.

EXAMPLE 4

A long consecutive electrolysis operation was conducted for one yearunder the same electrolytic conditions as Example 1 and the results ofelectrolytic performance obtained after one year were compared withExample 1, as tabulated in Table 2. From the comparison, it is foundthat for a long term stabilized operation, it is extremely effective toprepare the electrode surface to have the contact angle to water ofelectrode catalyst at 90 degrees or more.

EXAMPLE 5

Cathode Sheet E of porous structure was prepared in such way that PTFEdispersion (Mitsui DuPont Fluorochemical Co., Ltd. 30-J) that has beendiluted 10-fold with ethanol was applied with a brush on Cathode SheetC, followed by drying, and this process was repeated twice. The contactangle with the catalyst coating surface of Cathode Sheet E in porousstructure was improved to 91 degrees.

Then, the electrolysis test was conducted as in Example 1, the resultsof which illustrated the concentration of ozone in the anode gas: 11.4Vol. %, the concentration of hydrogen gas in the anode gas: 0.15 Vol. %,and the cell voltage: 3.4 v. Viewed from these results, it is found thatwhen said angle reaches 90 degrees or more, the cell voltage shows norising, the concentration of hydrogen in the anode gas decreases below0.15 Vol. %, and the purity of anode gas is extremely high.

From Example 5, it is demonstrated that when PTFE dispersion is appliedto the surface of Cathode Sheet C wherein the contact angle was 77degrees and the hydrogen concentration in the anode gas was 0.55 vol.%,the PTFE concentration on the surface is increased and the contact angleis also increased to 91 degrees, and then, the hydrogen concentration inthe anode gas can be suppressed to 0.15 vol.%.

Table 1 shows the results of said Examples 1, 2, 3, and 5 andComparative Examples 1, 2 and 3. From these results, it has been proventhat when said contact angle becomes 90 degrees or more, the cellvoltage does not show any significant rising, the transfer of hydrogengas or oxygen gas generated in the cathode or the anode into the counterelectrode through the cation exchange membrane is suppressed, the gaspurity is improved and safety electrolysis operation is secured for along time.

TABLE 1-1 Composition and Effect of Water Electrolysis System inExamples and Comparative Examples Example 1 Example 2 Example 3 CathodeSheet A Cathode Sheet B Cathode Sheet A Anode Catalyst Lead Dioxide LeadDioxide Iridium Powder Cathode Catalyst Platinum-loaded CarbonPlatinum-loaded Carbon Platinum-loaded Carbon Mixture of cathodecatalyst PTFE Dispersion PTFE Dispersion PTFE Dispersion CathodeSubstrate or Sintered sheet of stainless- Carbon Paper, Sintered sheetCarbon Paper, Sintered sheet Cathode Current Collector steel fiber ofstainless-steel fiber of stainless-steel fiber Anode Gas Oxygen/OzoneOxygen/Ozone Oxygen Cathode Gas Hydrogen Hydrogen Hydrogen Contact Anglewith Cathode Surface Deg. 92 95 95 Hydrogen Concentration in Anode Vol.% 0.05 0.10 0.07 Gas Cell Voltage V 3.3 3.3 2.5

Table 1-2

TABLE 1-2 Composition and Effect of Water Electrolysis System inExamples and Comparative Examples Comparative Example 1 ComparativeExample 2 Cathode Sheet C Cathode Sheet D Comparative Example 3 AnodeCatalyst Lead Dioxide Lead Dioxide Lead Dioxide Cathode CatalystPlatinum-loaded Carbon Platinum-loaded Carbon Thermally-decomposedPlatinum Mixture of cathode catalyst PTFE Dispersion, Nafion PTFEDispersion, Nafion None (Registered Trade Mark of Du (Registered TradeMark of Pont) Solution Du Pont) Solution Cathode Substrate or CathodeCarbon Paper, Sintered sheet Sintered sheet of stainless- Porous CarbonPlate Current Collector of stainless-steel fiber steel fiber 2 sheetsAnode Gas Oxygen/Ozone Oxygen/Ozone Oxygen/Ozone Cathode Gas HydrogenHydrogen Hydrogen Contact Angle with Cathode Deg. 77 45 28 SurfaceHydrogen Concentration Vol. % 0.55 0.84 1.55 In Anode Gas Cell Voltage V3.2 3.1 3.4

TABLE 1-3 Composition and Effect of Water Electrolysis System inExamples and Comparative Examples Example 5 Cathode Sheet E AnodeCatalyst Lead Dioxide Cathode Catalyst Platinum-loaded Carbon Mixture ofcathode catalyst PTFE Dispersion, Nafion (Registered Trade Mark of DuPont) Solution + PTFE dispersion Cathode Substrate or Cathode CarbonPaper, Sintered Current Collector sheet of stainless-steel fiber AnodeGas Oxygen/Ozone Cathode Gas Hydrogen Contact Angle with Cathode SurfaceDeg. 91 Hydrogen Concentration in Vol. % 0.15 Anode Gas Cell Voltage V3.3

Table 2 shows the results of Example 4. As a result, it has been proventhat in case of said contact angle at 92 degrees, the cell voltage hasnot risen even after a long time operation of one year, and theconcentration of hydrogen in the anode gas has been kept at extremelylow levels, compared with the case of said contact angle at 77 degrees.

TABLE 2 <Example 4> Comparison of Effects of Start Up vs. One Year-AfterComparative  Example 1 Example 1 Cell Voltage (V) Start Up 3.3 3.2 OneYear-After 3.1 3 Ozone Gas Concentration Start Up 11 11.4 (Vol. %) OneYear-After 11.7 11.2 Hydrogen Concentration in Start Up 0.05 0.55 Anodegas (Vol. %) One Year-After 0.5 1.62

Table 3 and FIG. 3 show the relation of contact angle vs. hydrogenconcentration in anode gas of Cathode Sheet 4 among the results fromExamples 1, 2, 3 and 5, and Comparative Examples 1, 2, and 3. From theseresults, it has been proven that when said contact angle is 90 degreesor more, the concentration of hydrogen in the anode gas becomes 0.15vol. % or below and the purity of the anode gas is extremely high.

TABLE 3 Relation of Contact Angle vs. Hydrogen Concentration in AnodeGas Contact Hydrogen Conc. Angle in Anode Gas (Degree) (Vol. %) 92 0.05Example 1 95 0.1 Example 2 95 0.07 Example 3 91 0.15 Example 5 77 0.55Comparative Example 1 45 0.84 Comparative Example 2 28 1.55 ComparativeExample 3

As mentioned above, the water electrolysis system by the presentinvention can suppress the transfer of hydrogen gas or oxygen gasgenerated at the cathode or the anode into the counter electrode throughthe cation exchange membrane, improve the gas quality and achieve safeelectrolysis operation for a long time.

This application claims the priorities of Japanese Patent Application2007-116670 filed Apr. 26, 2007 the teachings of which are incorporatedherein by reference in their entirety.

1. A water electrolysis system having an anode catalyst layer containinganode catalyst and a cathode catalyst layer containing cathode catalysttightly attached, respectively, to each surface of a solid polymerelectrolyte membrane comprising a cation exchange membrane, wherein atleast one catalyst layer of said anode catalyst layer and cathodecatalyst layer comprises a porous structure of anode catalyst or cathodecatalyst dispersed in fluorine resin containing resin, featuring thesurface of the anode catalyst layer or the cathode catalyst layer beinghydrophobized and the water contact angle with the surface of the anodecatalyst layer or the cathode catalyst layer of said porous structurebeing 90 degrees or more.
 2. A water electrolysis system according toclaim 1, in which the fluorine resin containing resin used for saidporous structure is polytetrafluoroethylene.
 3. A water electrolysissystem according to claim 1, in which the catalyst layer comprising saidporous structure is a cathode catalyst layer containing cathodecatalyst.
 4. A water electrolysis system according to claim 2, in whichsaid cathode catalyst layer comprises a porous structure of platinum orplatinum-loaded carbon grain dispersed in fluorine resin containingresin.
 5. A water electrolysis system according to claim 1, in which theporous structure comprises a porous metal plate or a sintered sheet ofmetallic fiber having on its surface anode catalyst containing leaddioxide or iridium in said anode catalyst layer.
 6. A water electrolysissystem according to claim 3, in which the catalyst layer comprising saidporous structure has a layer coated with fluorine resin layer on thesurface of said cathode catalyst layer.